@Elfs It looks like this mods calculates a new multiplier every time you start up the map though it is the same day as last time played.
If you save the lasDay number in yieldFluc.xml and then load it in to your lastDay variable, then that wont be a problem any more.
Save it: setXMLFloat(file, "yieldFluc#lastDay", yieldFluc.lastDay) after line 19.
Load it: yieldFluc.lastDay = getXMLFloat(file, "yieldFluc#lastDay") or -1 after line 29.
I don't know if it's on purpose you do it this way, if it is then sorry for disturbing you with this it's just a sugestion.
Also there could be a better way of doing this, but this way works.
Thanks for making this mod.